Question

Describe clearly how you would apply the concepts of pH control of natural waters and redox chemistry to precipitate or remove dissolved metals from acid mine water. Explain how pH and redox potential can be manipulated to remove different metals from solution. The metals may include: iron, copper, lead, zinc, arsenic, chromium

Answer 1

Standard Electrode Potential

Fe^​2+/ Fe,E^o = - 0.44 V , Fe³⁺/Fe, E^o= -0.036 V, Cu²⁺/Cu, E^o= +.0.34 V, Pb²⁺/Pb, E^o = -0.126 V, Zn²⁺/Zn, E^o = -0.76 V,

As³⁺ / As, E^o = and Cr³⁺/Cr, E^o = -.0.74 V

Arsenic is removed by adsorption rather than using electrochemical phenomenon.

Removal of Cu, Pb and Zn at pH 5, at 100-200 mg/litre in cation exchangers

Zn, Cd, Cu and Pb in ion exchanger at pH 5 and at 200 ppm

Cr is removed biosorption and ion exchange at pH < 6 and at 10-100 ppm metal solution

Fe is removed using Fluidized Bed Reactor at pH 2 and at 500 mg /L

The order in which metal ions get reduced and metals separated from solution containing metal ions, is

Zn ²⁺ , Cr ³⁺, Fe ²⁺, Pb ²⁺ Fe ³⁺ Cu ²⁺

Competing metal ions at a particular pH can be removed by using over-potential technique.

Solubility product , ionic product and common ion effect if applied wisely. separation of metal ions as insoluble metal salts is much easier.